Hybrid electrical grids, including the localized shipboard electrical grids used in marine vehicles, represent an area of increasing interest and rapid development, not least because these grids can significantly reduce greenhouse gas emissions as a consequence of their potentially greater efficiency. In this context, a “hybrid” electrical grid includes one or more generation sources and one or more energy storage systems. The energy storage system(s) provide a mechanism to store some of the energy produced by the generation source(s) and/or obtained through one or more regenerative processes, and to provide some or all of the stored energy to the grid, e.g., to meet peak loading demands and/or to smooth out the demand variations imposed on the generation source(s).
When a hybrid electrical grid includes more than one type of energy storage unit, e.g., a battery-based energy storage unit and a capacitor-based energy storage unit, the energy storage system represented by these different types of energy storage units may be referred to as a hybrid energy storage system. While use of different types of energy storage units in a hybrid energy storage system offers the promise of greater system efficiency, or at least greater operational flexibility, these benefits come at the expense of increased complexity. Put simply, the promised benefits are not obtained absent intelligent control of the hybrid energy storage system. Moreover, there exists a real risk that the increased complexity and/or expense of controlling hybrid energy storage systems will outweigh the potential benefits of such systems.
For example, known approaches to power-sharing control for hybrid energy storage systems extend to cases involving energy storage units of two different types. Control in this conventional context relies on either load power or load current measurements and these measurements often are difficult to obtain when the hybrid electrical grid includes distributed loads. This drawback notwithstanding, known approaches include filtering a measurement signal to obtain a filtered signal component for controlling an energy storage unit of a first type, and a remaining signal component for controlling an energy storage unit of a second type.
While the above approach is effective in the limited contexts where it is applied, it is not readily adaptable to hybrid energy storage systems involving multiple types of energy storage units. Moreover, the above conventional approach is ill suited for hybrid energy storage systems involving distributed loads, possibly on different buses, where local bus measurements may be critical for optimal operation of the various energy storage units.